This work is licensed under a Creative Commons Attribution 4.0 International License

Muthanna Journal of Engineering and Technology, Vol. (13), Issue (2), (2025)

Muthanna Journal of Engineering and Technology

Website: https://muthjet.mu.edu.iq/

Submitted 29 March 2025, Accepted 10 May 2025, Published online 16 May 2025

Review on Reinforced Concrete Slabs behavior with Presence

of Openings

Nameer M. Jawad Al-Quraishy a, Yousif J. lafta b, Thaer M. Saeed

Alrudaini c

a,b,c Department of Civil Engineering, Collage of Engineering, University of

Basrah, Basrah, Iraq

*Corresponding author Email: [email protected]

DOI:10.52113/3/eng/mjet/2025-13-02-/23-44

Abstract

Openings in slabs are an important issue that needs to be studied carefully because these openings have an important effect

on load capacity and the general behavior of RC, reinforced concrete slabs. This research aimed to review the previous studies

that highlighted the impact of presence of opening in RC one-way and two-way slabs in addition to a review of four important

codes, ACI Code, British Standard, Canadian Standard, and European Standard, that focused on this topic. The review of

previous studies is divided into two sections, the first is the effect of opening in one-way slabs and the second is the effect of

opening in two-way slabs. These studies produced that the opening size and position significantly affect load capacity,

flexural, shear resistance, and deflection of slabs. The opening in the slab reduces the load capacity and shear resistance by

reducing the concrete mass of the section where the concrete is cut. The flexibility of that slab is reduced in the existence of

the opening caused by reinforcement cutting. The effect of opening can be reduced by using additional reinforcements or by

using CFRP, carbon fiber reinforced polymer-strengthening.

Keywords: Review; RC; Opening; slabs

1. Introduction

Due to a large number of service requirements for construction buildings, nowadays, openings are intensely needed in slabs

and even in beams, such as openings for electrical cables, sewer, and cooling pipes or ducts or openings for stairs, elevators,

or even for architectural purposes. Care shall be taken while incorporating these openings. Some are before construction

which can be planned and some are after construction.

There are many different ways of restrengthening of opening in slab. These ways, are conducted internally before casting by

adding additional reinforcements around the opening, externally by using carbon fiber reinforcement with casted and cut

slabs, or by using both strengthening methods.

Several studies have been conducted on reinforced concrete slabs with openings to highlight the effect of opening on the

shear, flexural, and general behavior of slabs in addition to the effect of size, shape, and position of opening on these slabs.

This research will review the limitations and conditions of ACI code, BS code, ES code, and CSA code in addition to many

researches that studied the effects of openings in one-way and two-way slabs.

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2. Opening in slabs according to different codes

2.1. Openings in slab systems according to ACI 318-19 [1]

ACI Code, (8.5.4.1) shows that the openings in slabs of any size are allowable if they satisfy the requirements of strength

serviceability, and deflection. Alternatively, (8.5.4.2) shows that the opening is allowable in a slab system without beams if

satisfied the following:

a- If the opening was in the area of middle strip intersections, the missing reinforcements should be

compensated.

b- If the opening was in the area of the column strip intersection, the width of this opening shall not exceed

(1/8) of the column strip in each span. In addition, the interrupted reinforcements should be compensated

on the sides of the opening.

c- If the opening was in the intersection of the column strip and middle strip, the interrupted reinforcements

shall not exceed (1/4) in either strip. In addition, the interrupted reinforcements should be compensated on

the sides of the opening.

d- for slabs with shear heads, if the opening was in the column strip and close to the concentrated load or

reaction area by less than (4h), a portion of bo enclosed by straight lines projecting from the centroid of the

column, concentrated load or reaction area and tangent to the boundaries of the opening shall be considered

ineffective. The locations of the effective portions of the critical section near typical openings and free edges

are shown by the dashed lines in Figure (1).

Openings shown are located within 4h of the column periphery.

Fig. 1: Effect of openings and free edges (effective perimeter shown with

dashed lines). [1]

2.2. Openings in slab systems according to British Standard 8110-1 [2]

According to British Standard (8110-1), (3.7.5), the opening should not override the column head and openings should

formed with beams on all sides to transfer the loads to columns, otherwise should satisfy the following:

1- Opening in column strip: the greatest dimension of opening, in the direction than in line with the center-line of

the slab, should not more than 0.4 of length, and the design moments distributed between the remaining structure.

2- Opening located in the intersection of column strips: the largest dimension of the opening should not exceed one-

tenth of the column strip and the remaining section area is capable of carrying the design moments. In addition, the

design shear should be reduced if appropriate.

3- Opening located in the intersection of column strip and middle strip: the largest dimension of the opening should

not exceed one-quarter of the column strip and the remaining section area is capable of carrying the design moments.

In section (3.7.7.7), the effective perimeter is modified to allow for openings: If the location of the opening in the slab is at a

distance less than six times the effective depth from the border of the concentrated-load, then that part that is enclosed by

radial extensions from the center of the load is to the openings is considered not effective, as shown in figure (2).

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Fig. 2: Shear Perimeter of the slab with opening [2]

2.3. Openings in slab systems according to Canadian Standard A23.3 2019 [3]

In CSA (A23.3 2019), the opening in slabs had been considered according to the following:

1- In section (13.3.3.4) slab openings: If the distance between the opening and the concentrated load or load area is

less than ten times the slab thickness or the location of the opening is in the column strip, then the area enclosed by

the projections from the center of the load or load area and tangents to the boundaries of the opening considered as

ineffective area.

2- In section (13.5.4): opening in slabs of any size is allowable if the slab system analysis shows that the factored

resistance of the slab is at least equal to the factored loads in accordance with load compensations, load factors and

factored resistance mentioned in (8.3), (8.4) and (13.3.3.4).

3- In two-way slabs without beams, openings could be done without the requirements given by (13.5.4) if met the

following:

a- If the opening was located at the intersection of middle strips, the reinforcement amount of the slab without

opening should be maintained.

b- If the opening was located in the intersection of column strips, then the interruption should be equal to or less

than (1/8) of the column strip’s width in either span and the reinforcement amount that was missed should be

added adjacent to the opening sides.

c- If the opening was located in the intersection of the column strip and middle strip, then the interrupted

reinforcements should be equal to or less than (1/4) of reinforcements of either strip and the amount of the

reinforcement that was missed should be added adjacent to the opening sides.

d- The requirements of shear resistance, specified in (13.3.3.4), should be satisfied.

2.4. Openings in slab systems according to Europe Standard EN 1992-1-1 [4]

In section (6.4.2), Load distribution and basic control perimeter, the European Standard (1992-1-1_2004) mentioned the

openings in (6.4.2) the load distribution and basic control parameter. If the distance between the opening and the load area

does not exceed (6d), then the openings near the loaded areas, the area that is enclosed by the lines tangent to the opening

and drawn from the center of the load area is considered ineffective.

Fig. 3: Control perimeter near an opening according to ES (1992-1-1_2004) [4]

3. Previous researches that dealt with the issue of openings in slabs

There are a lot of researches that study the issue of the effect of openings in concrete slabs. These researches have dealt with

the study of slabs on the basis of the two main types of slabs, which are one-way slabs and two-way slabs. These researches

concentrated on the effect of opening size and location on the behavior of the slabs in terms of flexural, shear, and general

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behavior under concentrated or distributed loads. Table (2) illustrates the details of the slabs and openings of all studies with

failure mode and ultimate load reduction caused by the presence of opening.

3.1. Effect of opening in one-way slabs

Hereunder a summary of studies that dealt with the behaviour of one-way slabs:

Paolo Casadei et al, 2005 [5]; produced a field test of strengthening cut-out opening in a one-way slab. What distinguishes

this research is that the slabs that were tested are of a building that is scheduled for demolition. Openings were created in

a one-way flat slab in the negative moment region and positive moment region. Then, these openings were strengthened using

CFRP sheets with different schemes of strengthening. Six specimens were conducted as shown in Figure 4.

Fig. 4: Specimens of the building scheduled for demolition [5]

The details of opening and strengthening are shown in figure (5).

Fig. 5: Openings and strengthening schemes [5]

The tests explain that the strengthening using carbon fiber reinforcement polymer sheets significantly improved the load

capacity related to slab with opening, which approximately reached 30%. On the contrary, a shear failure occurred when the

opening was produced in the positive region and the strengthening with CFRP sheets is not effective in this region.

Koh Heng Boon et al, 2009 [6]; studied the behavior of a one-way slab with an opening. Five specimens were cast. One of

these slabs was cast as a controller specimen. the other four slabs were cast with an opening in the mid-region of the span.

One specimen had been casted with opening without any strengthening or additional reinforcements. One of the specimens

was with additional reinforcements parallel to the length and width of the opening. One other specimen was strengthened by

using additional diagonal reinforcement located at the edge of the opening. The last specimen was strengthened by using

reinforcement bars around the opening in addition to reinforcement bars that located diagonally in the corners of the opening.

Figure (6) shows the details of all specimens with opening and additional steel bars for strengthening.

Fig. 6: Strengthening bars of opening [6]

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The dimensions of all specimens were, 1100mm in length, 300mm in width, and 75mm in thickness, and the dimensions of

opening were 300mm in length and 150mm in width.

The results explained that the ultimate-load of the slab was reduced by 36% in the existence of the opening. However, the

best restrengthening way was by adding rebars surrounding the opening parallelly and diagonally, and adding only diagonal

bars did not give a significant change.

H. M. Seliem et al 2011 [7] had produced a case study on the restoration of flexural capacity of continuous one-way RC slabs

with cutouts. This research is distinguished by the fact that it was conducted on a real building that was scheduled for

demolition. In which, a field test was applied to one-way slabs cut out in the positive moment region to make an opening

with dimensions (610*610) mm2 which represents 18% of the clear span. Five different slabs were chosen to make five tests

with the presence of three different strengthening ways. The three ways were, near surface mounted FRB, externally bonded

FRB, and externally bonded with anchors FRB.

The results show that the flexural capacity of the slabs had been significantly affected by the presence of openings. However,

the NSM CFRP was more efficient than EB CFRP plates without anchors. However, the slab resistance was not restored.

Kadhim et al, 2013 [8]; introduced a nonlinear finite element study of slabs with opening strengthening by either overlay

concrete or carbon fiber reinforced polymer sheets surrounding the opening. The numerical analyses depended on a previous

study of nine specimens. The dimensions of these slabs were 1200mmX250mmX100mm, one of them was cast with

an unstrengthened opening, and eight others were cast with an opening then strengthened by overlay concrete and CFRP

sheets surrounding the opening. The experimental study consisted of eleven slabs with dimensions of

3600mmX2400mmX150mm simply supported and tested with two-line loads. The opening is cut with dimensions of

1200mmX800mm and strengthened by overlying concrete or CFRP sheets.

The results of this study concluded that the ultimate-load capacity was increased by 10% to 60% when CFRP sheets

were used. In addition, the use of CFRP sheets gives better efficiency than overlay concrete.

Ali M. Al-hafiz et al, 2013 [9]; in their paper, Flexural Strength of Reinforced Concrete One-Way Opened Slabs with and

without Strengthening, investigates a new strengthening technique to enhance the behavior of one-way slabs with openings.

Fifteen RC slabs were investigated in this paper. The opening was made in the middle of the span with dimensions of 75mm

for each side. A steel plate with steel connectors was used in all sides of the opening for strengthening. The variables were

the slab thickness and steel thickness. Were the slab thickness used were, 40mm, 60mm, and 80mm. The steel thicknesses

used were 2mm, 4mm, and 6mm. All sabs had the dimensions of 1100mm length and 400mm width. The steel plate used for

reinforcing the opening is shown in Figure (7).

Fig. 7: Specimens and steel plate used for strengthening [9]

Test results show that the ultimate loads with an opening without strengthening had been reduced by 23% to 27% if they

compared with slabs that have no opening. While the ultimate loads with strengthened openings had been reduced by 3.5%

to 23% in comparison with slabs without openings.

Qusay W. Ahmed 2018 [10], in his paper, worked on the behavior of reinforced concrete one-way slabs with openings. One

specimen was cast as a controlled specimen without any opening. Three specimens were implemented to have a square

opening in middle of the span with dimensions of 150mm, 200mm, and 250mm sides. Two specimens with rectangular and

circular openings. Two specimens were cast with additional strengthening bars surrounding the opening and diagonally at

the corners of the opening respectively. The last specimen was cast with double reinforcement surrounding the opening.

The results showed that an opening of 200mm*200mm reduced the load capacity by about 50%. The slab with strengthening

rectangular and diagonal bars showed a significant enhancement in load capacity and the additional diagonal bars showed no

significant effect.

Salman et al, 2018 [11] in their experimental work they studied the behaviour of slabs that have an opening. Six specimens

were prepared with dimensions of 1.5m length, 0.5m width, and 10cm thickness. One specimen was cast without an opening

and five specimens were cast with a (20cm*20cm) opening located at the center of the slab. CFRP is used for strengthening

the opening edges. The main parameters were the area, width, and length, of the CFRP and the presence of the opening.

Figure (8) explains the details of specimens.

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Fig. 8: Opening in slabs with different sizes. [11]

The results displayed that the load capacity was influenced by a 50% decrease in the presence of an opening in comparison

to the controlled specimen. The specimens that were strengthened by CFRP sheets recorded an increase of about (24-92) %.

The deflection at the mid zone was also reduced by about (40-62) % in the presence of CRFP. Finally, noticed a cracking

width reduction by about (44-95) %.

Abd-Elhakim A. Khalil et al, 2019 [12]; in their research studied the behavior of a cantilever slab with an opening. Seven

specimens were cast in the dimensions shown in the Fig (9).

Fig. 9: Control specimen details [12]

One of these specimens casted without opening as shown in Figure 9. While the other specimens were cast with different

locations and dimensions of the opening. Table (1) shows the details of openings and reinforcements.

Table 1: Openings and reinforcements details [12]

Group

Slab

Fcu (MPa)

Opening Shape

Internal reinforcement (mm2)

D10@160mm

SB-400-200

D10@160mm

SB-200-400

D10@160mm

SB*-400-200

D10@160mm

SM-400-200

D10@160mm

SM-400-400

D10@160mm

SM-200-400

D10@160mm

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Results show that the opening in the cantilever slab causes a significant reduction of the flexural and stiffness of these slabs.

The slab that has an opening closer to the support was the most affected slab.

Aman et al, 2020 [13], studied the performance of RC slabs with opening strengthened by CFRP sheets. In this research, ten

slabs with dimensions of 1000mmX530mmX25mm were cast. One controlled slab without opening and nine other slabs were

cast with the opening of area equal to 5%, 10%, and 20% of slab area. The strengthening applied was one ply, two ply, and

three plies of CFRP sheets around the opening as shown in Figure (10).

Fig. 10: Specimens and strengthening modeling [13]

The results indicated that the opening strengthened by three layers gives a highest resistance to the load. In addition, the

deflection is reduced by increasing the CFRP ply.

Nalan Kaya and Ozgür Anil, 2021 [14], tested seven specimens to predict the load capacity of one-way steel reinforced

concrete slabs. A comparison between experimental and theoretical, finite element analysis, study also had been done. The

specimen’s dimensions and the openings dimensions are explained in Figure (11).

Fig. 11: opening dimensions and locations.[14]

In addition to the specimens shown in Figure 11, one specimen without an opening had been cast for comparison. The

experimental results closely matched the theoretical ones in terms of displacement, ultimate-load, the stiffness, and the mode

of fail. The results showed a large amount of reduction of the ultimate load capacity in the specimen when the opening is

located in the region that has a high moment in comparison to slabs when the opening is located in its high shear area. The

ductility and energy dissipation showed a higher reduction. The opening which in the high shear area led to a brittle-failure

because of cracks occurring. The final results of this research indicated that the openings in the one-way slabs significantly

reduced the load capacity of that slabs.

Moataz Gamal et al, 2021 [15], performed a study of strengthening of continuous RC slab with opening using near-surface

mounting strengthening. Seven slabs with openings were cast with dimensions of 2400 mmX500 mm and thickness of

100mm. One of these slabs were used for comparison in which it has no any opening and the other slabs were casted with

different situations of openings and strengthening methods. Two points load centered at each span which length of 1100mm.

Ultimate load, crack width and mode, failure mode, and deflection were discussed in this research. A significant enhancement

of ultimate load was done by using near surface mounted steel bars ranging between 156% to 225%. Steel bars were more

effective in comparison with CFRP bars for strengthening the region of negative moments.

Abas Golham et al, 2023 [16], studied the flexural behavior of a one-way slab reinforced by glass fiber-reinforced polymer

GFRP bars with an opening strengthened by carbon fiber-reinforced polymer CFRP sheets. Five slabs were constructed for

an experimental flexural test. The dimensions of all slabs were: 750mm width, 2650mm length, and 150mm thickness. One

of these slabs was cast without any opening as a reference one. Four slabs were cast with square and rectangular openings

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with dimensions of 250mm*250mm and 250mm*500mm respectively. These openings were reinforced by CFRP sheets as

in Figure (12).

Fig. 12: Specimens Details [16]

The presence of an opening in the mid-span of the slabs reduced the failure load by about 41% for rectangular openings and

about 43% for two square openings in comparison to the slab without opening. The existence of CFRP sheets around

the opening has improved the slab’s capacity by about 44% to 52% in terms of flexural load and stiffness by about 95% to

101% and decreased the deflection by about 45% to 56%.

Al Kallas et al, 2023 [17], conducted an experimental program to evaluate the effect of opening in a one-way ribbed slab.

Four specimens were made of three ribs divided into one ribbed slab without an opening, one ribbed slab with an opening

without strengthening, and two ribbed slabs with opening an and strengthened by using carbon fiber reinforced polymer

sheets. The dimensions of all slabs were 2600mm in length, 825mm in width, and 175mm in depth. The opening was in the

flexural region of the slab with a side length of 400mm. The middle rib was cut included by the hole. Figure (13) explain the

details of the specimens with opening and strengthening.

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Fig. 13: Ribbed slab specimen with opening and strengthening details. [17]

After analyzing the four specimens to conduct the peak load for these slabs, the test concluded the following:

- The failure was a flexural zone failure for control and for the slab that has an opening without CFRP-strengthening.

The failure of strengthening slabs was due to the rupture of FRP sheets combined with the concrete cover.

- The slab with opening capacity was reduced by about (38%) compared with the control slab without an opening

because that opening was in the flexural zone and one rib was cut by this opening. In other words, only two ribs still

working.

- The strengthening by CFRP and GFRP sheets restored the slab’s load capacity. In addition, this strengthening

enhanced that capacity by about 2% to 8% in comparison with the control specimen without opening.

The stiffness of strengthened slabs with an opening enhanced by about 35% to 48% in comparison with unstrengthened slabs

with an opening. Also, energy dissipation reduced by about 11% to 19%.

3.2. Effect of opening in two-way slabs

Piotr Rusinowski, 2005 [18], in his thesis, discussed the behavior of two-way slabs with an opening under a distributed load.

The slabs were divided into two groups. The first group had been cast with openings and reinforced by steel bars. The second

group had been cast without opening and then the openings were sawed up and strengthened with Carbon Fiber Reinforced

Polymers (CFRP). All slabs are square with dimensions (2.6*2.6*0.1) m3. Uplift in the corners is allowed in boundary

conditions and because of that, the slab reinforced only in the bottom. There are two types of openings which named “small

hole” with dimensions (0.85*0.85) m2 and “large hole” with dimensions (1.2*1.2) m2 as shown in Fig (14). The opening

location was in the center of the slab.

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Fig. 14: Rusinowski specimens details. [18]

The results of the experiment show that the opening made in an existing slab could be strengthened by carbon fiber reinforced

bands which is adequate and applicable. Whereas, the additional reinforcements in the corners of the opening, in the case of

a slab cast with an opening, didn’t obtain enough results and cannot be recommended in a actual structure.

Chee Khoon Ng. et al, 2008 [19], studied the effect of opening in in a square slab. The yield line analysis method is used to

estimate the ultimate limit state.

Fig. 15: Square slab with a central-opening. [19]

Results show that the ultimate load capacity decreased by increasing the area of the opening. Also, the small openings, less

than 30% of the slab area, have an insignificant influence on the load-capacity. In addition, the very significant effect of the

opening in the slab occurs when the size of the opening is equal to or more than 50% of the slab dimensions.

Luaai Muhammed Abbas [20], 2011 delt with the problem of an opening in two-way RC slabs. In which, CFP, carbon-fiber-

polymer, is used instead of normal steel reinforcement. Eleven specimens of simply supported square slaps had been used

with openings in the center of these slabs. The controlled specimens are two slabs with openings reinforced by steel bars and

the remaining slabs reinforced with carbon fiber-reinforced polymer bars. The details of slabs and openings are shown in Fig

(16). The results show that the opening significantly affects the behavior of slabs in general and the slabs reinforced with

FRP bars deflected more than slabs reinforced with steel bars by a percentage of (21% - 29%).

Fig. 16: Experimental work specimens details. [20]

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EL-Shafiey et al in 2012 [21], studied the Behavior of Flat Slabs with Openings Adjacent to Columns. This study investigates

the punching capacity of a flat slab that has an opening adjacent to a column experimentally and theoretically. Seven

specimens of flat slab prepared with dimensions (1,700 x 1,700 x 150 mm) and loaded up to failure. One specimen

was constructed without opening as a controlled specimen and the remaining six specimens were divided into two groups.

The first group of three specimens with opening in front of the column face. The second group of three specimens

was constructed with an opening at the column’s corner. Column dimensions, slab thickness, and steel reinforcements

remained constant in all specimens, and opening size and position worked as the main parameters. Figure (17) shows the

specimens and openings location and dimensions.

Fig. 17: Specimens details of EL-Shafiey et al. [21]

The results show that the opening located at the column face reduced punching capacity by about 26%, 48%, and 51%, while

the opening located at the corner of the column reduced punching capacity by about 21%, 30%, and 39%. in addition, the

distance away from the column by 1.5 times of slab thickness has an insignificant effect on the slab punching strength.

Anil et al in 2014 [22], investigated the effects of opening size and location on the punching shear behavior of two-way

reinforced concrete slabs. Eight specimens had been constructed with different variable locations of openings in addition to

one specimen without an opening for comparison. The dimensions of the slabs were (2m*2m*0.12m). A square column with

dimensions of (0.2m*0.2m) is located at the center of the slabs to apply load on it. The locations and sizes of openings

are detailed in Figure (18).

Fig. 18: Specimens details of Anil et al. [22]

The results proved that there is an inverse relationship between the resistance of the slab to shear stresses and the dimensions

of the opening. Increasing the dimensions of the opening reduces the slab’s shear capacity. In addition, the further the hole

is from the column, the greater the shear capacity of the slab.

Oukaili et al in 2014 [23], in their research The Punching Shear Strength of Reinforced Concrete Flat Plates with Openings

studied the behavior of slabs with openings in terms of punching shear of simply supported slaps. The size and place of

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openings were the test parameters. The dimensions of the slabs were 70mm in depth with 1000mm for both directions. The

cross-section of the column was 150*150 mm2, 200mm in height as shown in Fig (19).

Fig. 19: Reinforcement’s details of slab. [23]

Six specimens (XXX, SF0, CF0, LF0, CC0, and CF1) had been tested. The opening sizes and locations are explained in

Figure (20).

Fig. 20: Specimens details of Oukaili et al. [23]

The effect of openings was decreasing in shear capacity between 11.43% and 29.25% in comparison with the controlled

specimen and decreasing of stiffness between 0.31% and 83.00% depending on the dimensions and position of these openings

with respect to the column.

Taehun Ha et al in 2015, [24] studied The Effects of Openings on The Punching Shear Strength of RC Flat-Plate Slabs

Without Shear Reinforcement. The aim of this study is to investigate the effect of opening in the punching shear of a flat

plate slab without shear reinforcements. The parameters of this study are the position and number of openings. Eight slabs

were tested depending on punching shear resistance. The shear resistance of all specimens compared with the prediction of

several codes. ACI, CEB, and FIB codes were adopted. The results show that the reduction of the critical section due to

the existence of openings causes a reduction of punching shear. The most influencing shape, in shear resistance, is the L

shape. Figure (21) shows the details of specimens and their openings.

Fig. 21: Specimens details of Taehun Ha et al. [24]

Chkheiwer et al, 2017 [25] in their paper, studied the ability to use the wire mesh as a strengthening of two-way slabs after

making an opening in different shapes. Fifteen slabs were constructed with the dimensions of 0.8m*0.8 m and 95mm

thickness and high strength concrete was used. The specimens were divided into two groups depending on the opening shape.

The first group had a square opening and the second one had a rectangular opening. The opening of the first group

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strengthened by using wire-mesh with different situations. The opening of the second group strengthened by using steel-

fibers with multiple ratios. Figure (22) shows the specimens’ details.

Fig. 22: Specimens details of Chkheiwer et al. [25]

The test results show that the two strengthening methods increased the load capacity of the tested slabs. Also, the two ways

of strengthening show a reduction in cracks in the inside face of openings. In general, the wire mesh technique was more

effective than the other technique.

Hassan et al, 2017, [26] introduced a study of introducing an opening in two-way loaded slabs. The slabs were rectangular

with dimensions of 1.65mX1.65mX0.08m and the surrounding beam with dimensions of 250mmX100mm as shown in the

Figure (23).

Fig. 23: Control slab details of Hassan et al. [26]

Seven specimens were cast, one was without an opening, three were cast with an opening and three were cast without

an opening then an opening was cut during loaded with 30% of the total load. The openings of the last three slabs were

supplied by a plate frame welded to reinforcement bars after removing the concrete cover then the concrete and

reinforcements were cut. The shapes of the openings were square, rhombus, and circular.

The results show that the slabs supplied with steel frames behave like the slabs that are cast with openings. In addition, the

deflection of them was larger. However, the ultimate load of the slabs with existing square rhombic and circular openings

was reduced by 60% 64%, and 55% respectively in comparison with the control slab, and the ultimate load of the slabs with

produced square rhombic and circular openings was reduced by 67% 78% and 80% respectively in compare with the control

slab.

Esraa Jasim et al in 2018 [27] tested ten specimens with an opening in addition to a controlled specimen without an opening

in this thesis. the dimensions of all specimens were (800mm*800mm) with a thickness of 70mm. The openings used were

(100mm*100mm), (150mm*150mm), (200mm*200mm) and finally (250mm*250mm). One of these specimens had been

cast without opening as a controlled specimen. The reinforcement used was steel bars of 8mm diameter. Figure (24) shows

the specimens' details with their openings.

Fig. 24: Specimens details of Esraa Jasim et al. [27]

Results were evaluated under uniformly distributed load and all slabs were simply supported at their four edges. Each model

containing an opening was examined once without strengthening and once with strengthening by CFRP sheets. All specimens

were designed to fail in flexural. A decrease in ultimate load was observed in the slabs that have strengthened openings in

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comparison with the controlled slab especially slabs with an opening that has dimensions of (150mm*150mm) that has

(100mm*100mm) away from the corner edge. The results showed that the cracking load was enhanced by the use of CFRP

sheets by about (6.0%-77%), and (2.0%-40.8%) of the maximum load comparing with the specimens without openings.

Rasoul et al in 2018 [28], studied the punching shear of self-compacting slabs with opening. The constants of this experiment

were slab dimensions, concrete of self-compacting, and steel reinforcement ratio while the variables were the shape and

location of openings. The dimensions of all specimens were 450mmX450mmX40mm as shown Figure (25).

Fig. 25: Specimens details of Rassul et al. [28]

Three shapes of openings, square, rectangular, and circular were used and the location of these openings varied as shown in

Figure (25). The designing of these specimens took in consider to fail by punching.

The results show that the ultimate punching shear of the slab decreased by increasing the opening size and the corner square

opening introduced the larger ultimate punch-shear. Regarding circle opening, the ultimate punching of slab corner opening

is the largest and the lowest one is the opening in front of the load location.

Mahlis et al in 2018 [29], studies how the opening effect on the behaviour of a two-way slab. The total number of specimens

was ten. These specimens were divided into two groups. The first group consists of five slabs. One of them was cast with an

opening in the mid-span as a controlled slab without strengthening, three slabs with openings strengthened internally with

additional reinforcements surrounding the opening in multiple lengths and one slab with opening strengthened by CFRP at

the tensile side. The second group consists of five slabs containing one slab tested to estimate the load-capacity as a controlled

one. The other four slabs were loaded reaching the occurring of the initial-cracks. After that, they were strengthened with

CFRP ply and near surface mounting steel-bars, and then openings were cut out at the center of the span then the load

was applied till complete failure occurred.

The results showed that the presence of an opening affected the load capacity of slabs by about 18.2% and the additional

reinforcements enhanced the load capacity by about 15% to 51% and finally the strengthening by CFRP and near-surface

mountain steel bars also enhanced the load capacity but using longer anchorage at the opening sides is more recommended.

Mohamed, et al in 2019 [30] report in their paper the results of the evaluation of the effect of strengthening a slab that has an

opening using carbon FRB composite sheets. The investigation was conducted numerically for ten specimens. One of these

specimens was established with an opening without any strengthening. Nine slabs were conducted with an opening in the

tension side and strengthened by CFRP laminates. The variables were the CFRP laminate thickness and width. The

dimensions of the control slab are shown in Figure (26).

Fig. 26: Control specimen details. [30]

Finite-element analysis was used to examine the ten specimens and concluded that load carrying capacity of slabs with large

openings increased by increasing the amount of CFRP sheets for strengthening and also the ductility index increased by

decreasing the amount of CFRP sheets.

Sheta et al in 2020 [31] conducted a parametric study of ten RC flat slabs with different opening sizes and locations using

ANSYS19. The slab dimensions were 3000mmX3000mmX200mm with a load column in the center of the slab with

dimensions of 200mmX200mm. A control slab was without an opening, as shown in the Figure (27), and nine other slabs

with different sizes and locations of openings.

Muthanna Journal of Engineering & Technology

Fig. 27: Control specimen details. [31]

The slab punch-shear force decreased by 10% when the opening changed from 400mm to 600mm and the punch shear

capacity is in direct-proportion to the load column face distance regardless of the dimensions of the opening. In addition,

the punching shear strength of the slab with an opening of 600mm*600mm increased by about 8.7% in comparison with the

opening of 400mm*400mm placed away from column-face by distance [d].

Yooprasertchai et al in 2021 [32] investigated the capacity of punch-shear for flat-plate slabs which has an opening. This

study consists of 2-groups. The first group has seven specimens, each specimen has two openings unless one with no opening

for comparison. The opening shapes were square, circular, and rectangular in different locations. The second group consists

of one slab with no opening, to be a control one, and six slabs with four openings for each slab. The opening shapes were

also square, circular, and rectangular in different locations. The dimensions of the slab and locations of openings can be

noticed in Figure (28).

Fig. 28: Specimens of Yooprasertchai et al. [32]

It was found that the minimum effect of opening on shear capacity was the circle opening and the largest effect was by

rectangular one. Furthermore, to get less impact of shear force, the opening should be at a distance of four times the thickness

of the slab away from the column location. Finally, the increase in opening numbers causes a decrease in the shear capacity

of the slab.

Hussain et al, 2022 [33] studied the effect of opening in a two-way slab. Four types of fibers were used to investigate how

the fiber of concrete affects the flexural behavior of these slabs. The aim of the study is to indicate the properties of hardened

concrete and the flexural behavior of two-way slabs. Two slabs, one with an opening and the other without, had been cast as

Muthanna Journal of Engineering and Technology

a controlled specimen. Eight slabs had been cast with different types of fibers. All slabs had a dimension of

(800mm*800mm*100mm) and were reinforced with 12mm diameter steel bars. Figure (29) explains the details of specimens.

Fig. 29: Specimens details of Hussain et al. [33]

Loads were applied at the center of the slabs on a steel-plate with dimensions of 200mm*200mm.

Results showed that the hooked fibers improve the cracking load and the flexural ductility of slabs generally enhanced in

the presence of fiber.

Zuhair et al in 2022 [34] studied the effect of circular opening on flat plate slab. The two-way slab was cast with

dimensions of 750mm for both sides without opening and three other slabs were cast with different diameters of openings.

The openings were 60mm, 75mm, and 90mm. The results of the test show that the load-carrying capacity of the slabs

decreased by increasing the size of the opening. Where the load capacity of the slab that had a 60mm opening was reduced

by 15.4%, the load capacity of the slab that had a 75mm opening was reduced by 20%, and finally, the load capacity of the

slab that had a 90mm opening reduced by 23%.

Shiyal et al in 2022 [35] studied the punching shear of thirteen slabs with the presence of openings in different sizes. One of

these slabs was without an opening, for comparison, and six slabs have openings of the following sizes: (100*100, 150*150,

200*200, 250*250, 300*300, and 350*350) mm2. The dimensions of the slabs and the punching column details are in Fig

(30).

Fig. 30: Specimens details of Shiyal et al. [35]

The variables of these tests were the opening size and steel reinforcement of these openings. The results of this study show

that the carrying capacity, cracking load, and strain of concrete are significantly affected by the existence of an opening

despite the reinforcement surrounding the opening which redistributes the shear stresses and makes more propagation in the

slab section.

Abd-EL-Mottaleb et al in 2022 [36] studied the effect of opening on a multi-panel flat slab with CFRP bars. This paper has

an experimental study and theoretical study using ANSYS 19. The experimental study consists of a two-way slab, 1100mm

length, and 1100mm width, with an opening of 250X250mm. The numerical study consists of six square slabs. The slab

dimensions are 12000X12000mm and a thickness of 150mm with marginal beams of depth 500mm and width of 250mm.

Columns distributed at 4000mm clear span. The slabs are divided as follows: one flat slab without an opening, three slabs

with an opening of 1000X1000mm varied in location, one slab opening of 2000X2000 mm, and one slab with an opening of

dimensions 800X800mm.

Results show that the load capacity and stiffness reduced in the presence of an opening and increasing the opening dimensions

reduced the efficiency of the slab. The largest effect of opening is when the opening is in the corner of the panel.

Shoukry et al in 2023 [37] worked at the experimental study on continuous RC flat slabs with openings strengthened with

CFRP. Five continuous slabs with openings were cast in addition to one controlled slab without opening. The details of

specimens shown in Figure (31).

Muthanna Journal of Engineering & Technology

Fig. 31: Specimens details of Shoukry et al. [37]

Three of these specimens that have different sizes and locations of opening had been strengthened by sheets of CFRP. The

results show that the ductility, toughness, and failure modes have been affected by the existence of openings. The slabs

strengthened by (CFRP) show enhanced failure load and failure modes.

Milligan et al in 2023 [38] studied the influence of opening in punching shear of RC slabs with L-shape column.

The finite-element method was used to analyze the slabs using ANSYS19. Slab details are in Figure (32).

Fig. 32: Slab details of Milligan et al. [38]

Figure (33) shows the opening situations.

Fig. 33: Opening locations. [38]

The results of the test illustrate the following:

- Stiffness and connection capacity reduced by the existence of an opening.

- Openings between column flanges of L-shaped slab column connections have a minor impact on concentric

behavior.

- Forces transferred along the outer edge of L-shaped slab column connections.

- Load transferred through one-way shear in that area.

- Region between column flange is relatively ineffective for L-shaped slab column connections.

- ACI318-19 for L-shaped slab column connections is inaccurate.

- ACI 421.1R-20 could be used for the estimation of punching shear capacity of L-shaped slab column connections.

Hadi et al in 2020 [39] studied the effect of opening on square-slab and rectangular-slab. The shape of the slab and the shape

of the opening are the two factors of this study. The theoretical method used to analyze the specimens was the yield line

method. The results of this study can be concluding the following:

Muthanna Journal of Engineering and Technology

- The decreasing of the opening decreases the area load capacity.

- The rectangular opening has a larger effect than the square opening.

- The area load capacity of the rectangular slab is larger than that square slab by about 2.5 percent.

- A significant reduction in slab strength caused by reducing an opening in an existing slab.

3.3. A brief result for one-way and two-way studies

Comparisons can be drawn between all previous studies in Table (2) to get a general idea of the effect of openings in concrete

slabs. Table 2: Comparison between the previous researches:

Reference

Slab

Type

Slab Dimensions

in (mm)

L*W*t

Opening Size

in (mm)

L*W

Opening Zone

Strengthening

Type

Failure

type

ultimate

load

reduction

with

opening

presence

(%)

Paolo Casadei

et al, 2005[5]

One-way

1030*970*200

350*200

Middle strip and

column strip

CFRP

Flexural

and shear

Koh Heng

Boon et al,

2009 [6]

One-way

1100*300*75

300*150

Centre of the slab

Steel bars around

and diagonally on

the corners of the

opening

Flexural

H. M. Seliem et

al, 2011[7]

One-way

15748*3358*200

610*610

Middle strip

CFRP

Flexural

Majid

Mohammed et

al, 2013 [8]

One-way

1200*250*100

3600*2400*150

800*1200

Centre of the slab

CFRP

Flexural

Ali M. Al-hafiz

et al, 2013 [9]

One-way

1100*400*40

1100*400*60

1100*400*80

75*75

Centre of the slab

No Strengthening

Flexural

Qusay W.

Ahmed 2018

[10]

One-way

1400*450*90

150*150

200*200

250*250

Centre of the slab

Steel bars

diagonally at the

corners of the

opening and

additional

reinforcement

around the opening

Flexural

Wissam et al,

2018 [11]

One-way

1500*500*100

200*200

Centre of the slab

CFRP

Flexural

Khalil et al [12]

Cantilev

er slab

2550*830*120

200*400

400*400

Close to support

No Strengthening

Flexural

Aman et al,

2020 [13]

One-way

1000*530*25

160*160

230*230

320*320

Centre of the slab

CFRP

Flexural

Kaya et al,

2021[14]

One-way

3000*1000*150

300*300

400*400

500*500

High moment area

and high shear area

No Strengthening

Flexural

Moataz Gamal

et al, 2021[15]

Continu

ous One-

way

2400*500*100

Not mentioned

Near-surface

mounting

strengthening

Flexural

Golham et al,

2023 [16]

One-way

2650*750*150

250*250

250*500

Centre of the slab

CFRP

Flexural

Al Kallas et al,

2023 [17]

One-way

ribbed

slab

2600*825*175

400*400

Centre of the slab

CFRP

Flexural

Rusinowski,

2005 [18]

Two-

way

2600*2600*100

850*850

1200*1200

Centre of the slab

CFRP

Flexural

Ng. et al, 2008

[19]

Two-

way

5000*5000*D

4000*4000*D

3000*3000*D

From 0 to 0.9 of

slab length

Centre of the slab

No Strengthening

Flexural

Luaai

Muhammed

Abbas, 2011

[20]

Two-

way

1050*1050*75

250*250

330*330

Centre of the slab

No Strengthening

Flexural

EL-Shafiey et

al in 2012 [21]

Two-

way

1700*1700*150

200*200

300*300

400*400

Close to center-

load column

No Strengthening

Punching

shear

Muthanna Journal of Engineering & Technology

Table 2 (continued)

Anil et al, 2014

[22]

Two-

way

2000*2000*120

300*300

500*500

Away from or close

to center-load

column

No Strengthening

Punching

shear

Oukaili et al,

2014 [23]

Two-

way

1000*1000*70

150*150

225*225

Away from or close

to center-load

column

No Strengthening

Punching

shear

Taehun Ha et al

in 2015 [24]

Two-

way

2000*2000*180

Circle of 150mm

diameter

Away from or close

to center-load

column

No Strengthening

Flexural

Chkheiwer et al

in 2017 [25]

Two-

way

800*800*95

300*150150*150

25mm from the

edge of the slab

Wire mesh

And steel fiber

Flexural

Hassan et al,

2017 [26]

Beam

edge

two-way

slab

1650*1650*80

300*300

and circle of

300mm diameter

Centre of the slab

Steel frame at the

circumference of

the opening

Flexural

Esraa Jasim et

al, 2018 [27]

Two-

way

800*800*70

100*100

150*150

200*200

250*250

Centre of the slab

Steel bars at the

corner of the

opening and

CFRP

Flexural

Rasoul et al,

2018 [28]

Two-

way

450*450*40

75*75

75*150

And circle of

75mm diameter

Close to the edge of

the slab and in the

corner of the slab

No Strengthening

Punching

shear

Mahlis et al,

2018 [29]

Beam

edge

two-way

slab

1650*1650*100

300*300

Centre of the slab

Steel bars

surrounding the

opening and

CFRP

Shear

Mohamed et al,

2019 [30]

Two-wat

continuo

us flat

slab

16000*12000*300

12000*3000

Middle strip

CFRP

Flexural

Sheta et al,

2020 [31]

Two-

way

3000*3000*200

400*400

600*600

Close to the center-

load column

No Strengthening

Punching

shear

Yooprasertchai

et al, 2021 [32]

Two-

way

1000*1000*80

70*70

70*140

Circle of 70mm

diameter

Away from or close

to center-load

column

No Strengthening

Punching

shear

Hussain et al,

2022 [33]

Two-

way

800*800*100

150*150

Centre of the slab

Steel fiber

Flexural

Muthanna Journal of Engineering and Technology

Table 2 (continued)

Zuhair et al,

2022 [34]

Two-

way

750*750*75

Circle of 60mm,

75mm, and

90mm diameter

Close to center-

load column

No Strengthening

Flexural

Shiyal et al,

2022 [35]

Two-

way

1000*1000*70

100*100

150*150

200*200

250*250

300*300

350*350

100mm from the

central load column

Steel bars around

and at the corners

of the opening

Punching

shear

Abd-EL-

Mottaleb et al,

2022 [36]

Two-

way

1100*1100*D

12000*12000*150

250*250

800*800

1000*1000

2000*2000

Middle strip

And column strip

No Strengthening

Flexural

Shoukry et al,

2023 [37]

Two-

way

3000*3000*60

200*200

400*400

Column strip

CFRP

Punching

shear

Milligan et al,

2023 [38]

Two-

way

2160*2160*152

X*Y

Close to punching

column

No Strengthening

Punching

shear

Hadi et al, 2020

[39]

Two-

way

(3-5)*(3-5)*t

From 0 to 0.9 of

slab area

Centre of the slab

No Strengthening

Flexural

It is obvious that there are many researches that published in the literature that discussed the effect of opening on the

performance of slabs as well as the strengthening technique to compensate the reduction in slabs load carrying capacity. Most

of the studies have focused on either one-way single span slabs or two-way single span slabs. However, very few studies

have considered multiple span slabs or continuous slabs in which the redistribution of loads and the effect of openings on the

adjacent slabs were ignored. Also, studies are required to understand the effect of openings on beam-slab systems where the

openings are inserted at the face of beams. In addition, more studies are required to optimize the location of openings in slabs

with minimum effect on the performance.

4. Conclusion:

Based on the previous researches, reviewed previously, it can be concluded that openings in concrete slabs can affect the

overall behavior of these slabs significantly despite the methods used to restrengthen these slabs, as follows:

1- ACI 318-19 Code and the other codes recommended that the opening of any size is allowable if it satisfy the strength

and deflection requirements, unless there is a limitation should be followed. These limitations depend on the which

strip of slab that the opening located in. In all cases, a compensation of reinforcement should be provided around

the opening.

2- British standards recommended providing a beam on all sides of opening to transfer the loads to the column.

3- According to ACI code, the area that enclosed by straight lines projecting from the centroid of the column,

concentrated load or reaction area and tangent to the boundaries of the opening shall be considered ineffective if the

opening is at a distance less than 4h from that concentrated load or column. While the British Standard consider that

distance as 6d. Canadian standard considered that distance as 10h and European Code considered it as 6h. where h

is the slab thickness and d is the effective depth of the slab.

4- According to previous studies, the presence of openings in RC slabs has negative effect on the flexural, shear

resistance, and load-capacity of slabs. This effect is inversely proportional to the dimensions of the opening in

the slab. In which, the larger the opening, the more negatively it affects the slab performance.

5- Opening size has a direct proportion with the deflection, concrete strain, and crack width of concrete slabs.

6- In slabs that had a concentrated load or column, the load resistance of this slab is inversely proportional to the

closeness of this opening with that load or column. In which, all codes put a limit to the distance between the opening

and the load or column.

7- In the slab that has a beam in all directions, the effect of this opening will be larger if that opening is placed in the

middle of the slab.

Muthanna Journal of Engineering & Technology

8- Generally, the resistance of the slab with opening could be enhanced by adding an additional reinforcement or by

using carbon fiber reinforced polymers (CFRP) or both methods can be used to return the slab to its design strength.

9- Almost all published in the literature have focused on either one-way single span slabs or two-way single span slabs.

10- Very few studies have considered multiple span slabs or continuous slabs in which the redistribution of loads and

the effect of openings on the adjacent slabs were ignored.

11- Up to the knowledge of the authors there are no studies that considered the effect of openings on beam-slab systems

where the openings are inserted at the face of beams.

For future studies, it is recommended to optimize the location of openings in slabs with minimum effect on the performance

where several options are available.

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